Wireless power transfer (WPT) or wireless energy transmission is the transmission of electrical power from a power source to a consuming device using a time-varying electromagnetic field, without using solid wires or conductors between the transmitter of the power and the receiver. In near-field or non-radiative techniques, power is transferred over short distances (typically less than 10 cm) mostly by magnetic fields using an inductive, resonant or non-resonant, coupling between coils or, in some devices, by electric fields using capacitive coupling between electrodes. Applications are for instance electric toothbrush chargers, RFID tags, smartcards, chargers for implantable medical devices like artificial cardiac pacemakers and other small appliances.
A current focus is the development of wireless systems to charge batteries of mobile and handheld computing devices, such as cell phones, digital music players and portable computers without being tethered to a wall plug. The best known example is the wireless charging of mobile phones and other mobile devices, for which the Wireless Power Consortium published the Qi resonant wireless charging standard. The Qi resonant wireless charging standard specifies a standard for inductive electrical power transfer over distances of up to 4 cm of low power (about 5 W) or medium power (up to 15 W) using an inductive coupling between two planar coils, i.e. between a primary coil (transmitting coil or TX) in a charger to a secondary coil (receiver coil or RX) in e.g. a mobile phone or other mobile device. The inductive coupling may be non-resonant (for which Qi uses the term “inductive”) or resonant (for which Qi uses the term “resonant”). In case of a resonant coupling, each of the two coils is part of a respective resonant circuit, and both resonant circuits have about the same resonance frequency. Non-resonant charging is generally deemed to be suitable for systems where the distance between the two coils is less than their diameter, whereas resonant charging is deemed suitable for systems where that distance is larger than the diameter of the coils.
However, the known wireless charging systems requires a precise alignment, in position and/or orientation, between the primary coil and the secondary coil. Although Qi also specifies arrays of primary coils which provides for a wider charging area, still the secondary coil needs to be positioned on the surface of the charger and oriented precisely relative to the primary coil to enable an effective charging. Furthermore, in case multiple devices need to be charged they need to be posited parallel to each other on the surface in order to avoid shielding of a secondary coil by another secondary coil.